Backlight modules are often used in electronic products such as mobile phones and personal digital assistants. Often these backlight modules are installed in keypad assemblies to enable users of mobile phones and personal digital assistants to see the keypads in low light environments.
Light guide sheets are often installed in keypad assemblies to provide backlighting for a keypad. Usually light from one or more LEDs is transmitted into a light guide sheet at the periphery of the light guide sheet. After light from the LEDs is transmitted into the light guide sheet, light is reflected internally in the light guide sheet. A portion of the light transmitted into the light guide sheet leaves a top surface of the light guide sheet. The light leaving the top surface of the light guide sheet illuminates the keypad is directly above the top surface of the light guide sheet.
Securing a light guide sheet to a PCB (Printed Circuit Board) or a dome sheet can be problematic. One method used to secure a light guide sheet to a PCB or a dome sheet is to use adhesive to “stick” the light guide sheet to a PCB or a dome sheet. This technique does not always allow the light guide sheet to stay flat on a PCB or a dome sheet. For example, if the distance from an edge of a metal dome to an edge of a secured light guide sheet is too short, tension may be created at an edge of the light guide sheet. Due to the tension created by the metal dome and the secured light guide sheet at an edge of the light guide, the light guide sheet may separate or “lift” from the PCB or the dome sheet. In addition, if the light guide sheet is lifted along the side where LEDs are positioned, the amount of light transmitted into the light guide sheet may be reduced.
Another problem that may occur when the distance from a metal dome to an edge of a secured light guide sheet is too short is that a metal dome may be lifted from a PCB. In this example, the adhesive applied may be too strong. As a result, the metal dome will be lifted from the PCB while the light guide sheet remains secure.